Position of a moving platform such as a ship at sea or a moving land vehicle is generally determined using a Global Positioning System (GPS). Without GPS, dead reckoning is often used in which measurements of angular rotation rates and translational accelerations (using gyroscopes and accelerometers) are integrated to calculate position. This method of position determination is unavoidably subject to drift errors which grow with time, making it useful for accurately determining position only over relatively short periods of time.
Historically, position determination using celestial observations has been achieved using the sight reduction method which uses measurements of star altitudes off the horizon to compute lines of position, the intersection of which defines the estimate of ship position. This method requires either visual measurement of the horizon or inertial measurement of the local gravity vector as a benchmark for all star observations and is therefore highly inaccurate on a moving platform.
Position of a moving platform may also be calculated by triangulating an observer's position from three or more satellite position measurements. This method, however, is sensitive to knowledge of the absolute knowledge of the position of the satellites.
It would be desirable to have a geoposition determination scheme that is not susceptible to the accuracy issues that plague prior techniques.